Kevin Systrom

In April 2012, three months after topping last year's FORBES 30 Under 30 Social/Mobile list, Systrom sold Instagram to Facebook in a deal that was valued at $1 billion. Systrom and cofounder Mike Krieger had just launched it 16 months earlier. Now with more than 100 million users, Instagram is a key weapon in Facebook's battle with Twitter to be the primary photo app for mobile users.